1. Field of the Invention
The present invention relates to a multi-beam scanning optical system and an image forming apparatus using the same and, more particularly, to a multi-beam scanning optical system suitably used for an image forming apparatus applicable to any of monochrome image and color image, e.g., a laser beam printer or digital copying machine, which can attain a high operation speed and high recording density by using a light source means with a plurality of light sources.
2. Related Background Art
FIG. 7 is a sectional view (main scanning sectional view) of the main part of a conventional multi-beam scanning optical system in the main scanning direction.
Referring to FIG. 7, a plurality of light beams emitted from a multi-beam semiconductor laser (multiple light sources) 71 having a plurality of light sources (light-emitting units) are converted into substantially parallel light beams or convergent light beams by a collimator lens 72, and the light beams strike a cylindrical lens 74 while the cross-sectional area of the light beams is limited by an aperture stop 73. Of the light beams incident on the cylindrical lens 74, light beams in a main scanning cross-section emerge without any change. In a sub-scanning cross-section, light beams converge and are formed into almost linear images (elongated in the main scanning direction) on a deflecting surface (reflecting surface) 75a of an optical deflector 75. Note that each of the collimator lens 72, aperture stop 73, cylindrical lens 74, and the like forms one element of an incidence optical system. A plurality of light beams reflected/deflected by the deflecting surface 75a of the optical deflector 75 are formed into spots on a photosensitive drum surface 77 by an imaging optical system (fxe2x88x92xcex8 lens system) 76. By rotating the optical deflector 75 in the direction indicated by an arrow C, these light beams are scanned on the photosensitive drum surface 77 at a constant speed in the direction indicated by the arrow D (main scanning direction). With this operation, an image is recorded on the photosensitive drum surface 77 serving as a recording medium.
In such a multi-beam scanning optical system, to accurately control the write position of an image, a write position sync signal detection means is generally arranged immediately before an image signal write position.
Referring to FIG. 7, a bending mirror (BD mirror) 78 reflects, toward the BD sensor 81 side, a light beam (BD light beam) for detecting a write position sync signal for adjusting the timing at a scanning start position on the photosensitive drum surface 77. A slit (BD slit) 79 is located at a position equivalent to the photosensitive drum surface 77. A BD lens 80 serving as an imaging means makes the BD mirror 78 and BD sensor 81 optically conjugate to each other, thereby correcting a tilt of the surface of the BD mirror 78. The optical sensor (BD sensor) 81 serves as a write position sync signal detection element. Note that each of the BD slit 79, BD lens 80, BD sensor 81, and the like forms one element of a write position sync signal detection means 91.
Referring to FIG. 7, the timing at a scanning start position for image recording on the photosensitive drum surface 77 is adjusted by using an output signal from the BD sensor 81.
In this multi-beam scanning optical system, as shown in FIG. 8, if a plurality of light sources (light-emitting units) A and B are placed vertically in the sub-scanning direction, the distance between scanning lines on a scanned surface in the sub-scanning direction becomes much larger than the recording density. For this reason, in general, as shown in FIG. 9, the plurality of light sources A and B are placed obliquely in the sub-scanning direction with respect to the main scanning direction, and a tilt angle xcex4 is adjusted to accurately match the distance between the scanning lines on the scanned surface in the sub-scanning direction to the recording density.
If a relative wavelength error occurs in light beams emitted from the plurality of light sources, relative focus errors occur on the scanned surface in the main and sub-scanning directions in correspondence with the respective light-emitting units, resulting in a deterioration in image quality. For this reason, a collimator lens having undergone proper chromatic aberration correction is used to prevent relative focus errors in the main and sub-scanning directions from occurring even if a relative wavelength error occurs in light beams emitted from a plurality of light sources, thereby effectively preventing a deterioration in image quality.
In the multi-beam scanning optical system having the conventional arrangement described above, since a plurality of light sources are placed obliquely in the sub-scanning direction with respect to the main scanning direction, light beams emitted from the light sources A and B strike the deflecting surface of the optical deflector (polygon mirror) at positions spaced apart from each other in the main scanning direction, and are reflected/deflected by the optical deflector at different angles, as shown in FIG. 10. As a consequence, spots are formed on the scanned surface 77 at positions spaced apart from each other in the main scanning direction (beams A1 and B1).
In the multi-beam scanning optical system having this arrangement, therefore, image data are sent with a timing shift of a predetermined time xcex4T such that a light from a given reference light source is formed into an image on the scanned surface, and then a light beam from another light source is formed into an image at the same imaging position.
Referring to FIG. 10, when the timing shifts by xcex4T, the deflecting surface 75a is set at the angle of a deflecting surface 75b. At this time, a beam B2 is reflected/deflected by the deflecting surface 75b in the direction indicated by an arrow B2xe2x80x2, i.e., in the same direction indicated by an arrow A1xe2x80x2 as that of the beam A1, thereby matching the imaging positions of the respective spots to each other.
Consider a case wherein a focus error occurs in the main scanning direction due to some cause, e.g., a positional error between the scanned surface and the optical unit holding the optical system or a mounting error in mounting an optical component in the optical unit. Assume, in this case, that a normal position 77a of the scanned surface 77 shifts to a position 77b. In this case, as is obvious from FIG. 10, the imaging position of each beam shifts in the main scanning direction by xcex4Y.
In the prior art, when the imaging positions of light beams from a plurality of light sources (multi-beam semiconductor laser) shift in the main scanning direction, the printing precision and image quality deteriorate.
There are various factors that cause focus errors in the main scanning direction. It is very difficult to reduce all these factors to zero. Even if such adjustment is to be made, the adjustment process will cost much. In addition, recently, an optical system having an fxe2x88x92xcex8 lens made of a plastic material is often used in consideration of cost. A plastic lens is manufactured by injection molding. The surface precision of this lens is inferior to that obtained by polishing optical glass. In such plastic lens, in particular, errors tend to occur in some portions in convex forms with respect to design values, but errors tend to occur in other portions in concave forms with respect to design values. When focus errors are caused by such surface precision errors, it is impossible to correct the focus errors throughout the scanned surface. It is therefore difficult to correct a deterioration in image quality due to the occurrence of errors in the imaging positions of light beams from a plurality of light sources in the main scanning direction.
It is an object of the present invention to provide an multi-beam scanning optical system which can effectively reduce errors in the imaging positions of light beams from a plurality of light sources in the main scanning direction and obtain a high-quality image at high speed, without requiring any complicated adjustment, by properly setting the respective elements of the multi-beam scanning optical system, and an image forming apparatus using the multi-beam scanning optical system.
According to an aspect of the present invention, there is provided a multi-beam scanning optical system comprising light source means having a plurality of light sources spaced apart from each other at least in a main scanning direction, a collimator lens for converting a plurality of divergent light beams emitted from the light source means into substantially parallel light beams or convergent light beams, deflection means for reflecting/deflecting the plurality of light beams in the main scanning direction, a stop which is interposed between the collimator lens and the deflection means to limit a width of incident light beams, and an imaging optical system for forming the plurality of light beams reflected/deflected by the deflection means into images on a scanned surface and making the deflecting surface and the scanned surface substantially conjugate to each other in a sub-scanning cross-section, wherein letting S1 be an emission point interval between the plurality of light sources in the main scanning direction, f1 be a focal length of the collimator lens, L1 be a distance from the stop to the deflecting surface of the deflection means, f3 be the focal length of the imaging optical system in the main scanning direction, and N1 be the number of pixels per inch on the scanned surface in the main scanning direction, a condition given by             S1      xc3x97      L1              f1      xc3x97      f3        ≤            25.4      N1        xc3x97          1      4      
is satisfied.
According to another aspect of the present invention, the multi-beam scanning optical system further comprises write position sync signal detection means for controlling a timing at a scanning start position on the scanned surface by using one of the plurality of light beams reflected/deflected by the reflection means, and controls the timing at the scanning start position on the scanned surface with respect to one of the plurality of light beams emitted from the light source means.
According to still another aspect of the present invention, the multi-beam scanning optical system further comprises write position sync signal detection means for controlling a timing at a scanning start position on the scanned surface by using all the plurality of light beams reflected/deflected by the reflection means, and controls the timing at the scanning start position on the scanned surface with respect to the plurality of light beams emitted from the light source means.
According to still another aspect of the present invention, the write position sync signal detection means has a slit, and the slit can move in a direction of the center of a light beam incident on the slit.
According to still another aspect of the present invention, the write position sync signal detection means has a slit, and the slit can pivot within a cross-section substantially perpendicular to the center of a light beam incident on the slit.
According to still another aspect of the present invention, the plurality of light sources comprise monolithic devices.
According to still another aspect of the present invention, there is provided an image forming apparatus which comprises the multi-beam scanning optical system set out in the foregoing, a photosensitive member disposed on the scanned surface, a developing unit for developing, as a toner image, an electrostatic latent image formed on the photosensitive member by a light beam scanned by the multi-beam scanning optical system, a transfer unit for transferring the developed toner image onto a transfer member and a fixing unit for fixing the transferred toner image on the transfer member.
According to still another aspect of the present invention, there is provided an image forming apparatus which comprises the multi-beam scanning optical system set out in the foregoing and a controller for converting code data input from an external device into an image signal and inputting the signal to the multi-beam scanning optical system.